Project Summary Brain metastasis, the distant relapse of a cancer in brain originally from another organ, occurs in 30% of breast cancer patients and is one of the leading causes of their mortality rate. A major barrier for understanding and treating brain metastasis is the complexity of the microenvironment around the brain blood vessel which plays critical roles in metastatic progression, delivery and efficacy of chemotherapeutic drugs, and the side effects of chemotherapy. In addition, breast cancer shows distinct therapeutic responses and disease progression across individual patients and thus the clinical analysis and evaluation of the treatment options require much larger randomized samples of patients with long-term clinical history. However, animal models, the most commonly used platform for screening cancer treatment strategies, have the inherent limitation in efficiency for testing the vast number of possible drug combinations, setting aside the controversy on the degree of their clinical relevance to the human diseases. In this project, we aim to develop a reliable, clinically relevant in vitro experimental model of breast cancer brain metastasis for efficient screening of chemotherapy drugs. More specifically, we will reconstruct the complex microenvironment around the brain blood vessel within a hydrogel- containing microfluidic chip, complete with its 3D cellular network and the surrounding bloodstream simulated with microfluidics technology. The clinical relevance of this artificial yet native-like brain blood vessel environment will be confirmed through the breast cancer cells behaviors akin to the clinical data in terms of the response to anti-cancer agents. Our breast cancer brain metastasis model will be a time- and cost-efficient test bed for screening and identifying the optimal treatment choice for each patient as well as for predicting and preventing the risk of brain metastasis development in individual patients. The highly promising, new chemotherapeutic targets identified from this project will lead to preclinical studies in which our model can further serve as an effective experimental platform. Furthermore, by incorporating individual patients? cells into our in vitro system, we will eventually be able to build patient-specific models, opening up a possibility of truly customized treatment for each patient.